The invention pertains to the field of optical apparatus for dye lasers, and, more particularly, the field of translation of mirrors in a dye laser to change the position of the focal point.
In dye lasers, a narrow jet of fluorescent dye under pressure is formed in a laser cavity. The laser cavity is defined by a plurality of mirrors which reflect light emitted from the dye during lasing action. The light is reflected back and forth within the cavity and passes through the dye jet repeatedly. Another mirror called the "pumping mirror" resides outside the laser cavity. This mirror receives a beam of coherent excitation light from a pump laser and reflects this excitation light toward the dye jet as an excitation beam. The excitation beam may be of steady intensity (called continuous wave) or it may consist of pulses. The pulses can have any duration, repetition rate or light frequency. Maximum pulsed dye laser performance occurs when the excitation beam frequency (color) is close to the dye laser frequency and the repetition rate of the excitation beam matches the round trip time of the dye laser cavity (known as synchronous pumping). The purpose of the excitation light beam is to input sufficient energy to cause the dye molecules in the jet to jump to higher energy states. When the dye jet molecules fall back to their lower energy states either spontaneously or through stimulated emission, they emit photons of light all of which have the same frequency and the same phase. This emitted light is called coherent.
A laser uses this process in performing light amplification by stimulated emission. The mirrors which define the laser cavity serve to collect the photons emitted from the dye jet and focus and reflect this energy back through the dye jet. As the photons pass through the dye jet, they trigger further emissions of photons of the same frequency and phase. That is, light emitted from the dye jet when dye molecules drop to lower energy states is reflected back and forth in the laser cavity and causes further dye molecules to drop to lower energy states to emit further photons. This process is called stimulated emission.
There are multiple mirrors in the laser cavity, two of which are placed on either side of the dye jet. These two mirrors are curved and serve to reflect and to focus the emitted light from the dye such that the narrowest point in the beam of light reflected between these two mirrors is within the dye jet. This concentrates the maximum amount of energy from the emitted photons in the dye jet and triggers the maximum number of stimulated emissions.
It is important to efficient laser action that the excitation light which pumps the dye molecules to higher energy states be focused properly by the curved pump mirror. The focus must be such that the beam of excitation light has a focal point within the dye jet. There also must be a maximum amount of overlap between the focal point of the excitation beam and the volume of the dye jet which is illuminated by the laser cavity beam being reflected between the two curved mirrors on either side of the dye jet. This causes the greatest efficiency in exciting dye molecules to create the necessary population inversion, i.e., a greater number of dye molecules in higher energy states than in lower energy states. The degree of overlap is related to the efficiency of creating stimulated emission. In other words, when there is maximal overlap, the photons being reflected within the laser cavity pass through the area of the dye jet which has the most number of excited dye molecules. This maximizes the number of stimulated emission photons released.
To maximize the efficiency of creating the population inversions and the efficiency of causing stimulated emission, it is important to be able to move the focal points of the pump mirror and the two mirrors in the laser cavity on either side of the dye jet. By being able to move the focal points of the mirrors on either side of the dye jet, the narrowest portion of the beam within the laser cavity may be placed within the volume of the dye jet. By being able to move the focal point of the pump mirror, the focal point of the excitation energy may be placed within the intersection of the laser cavity beam the dye jet.
In the prior art, the two mirrors on either side of the dye jet and the pump mirror were translated to move their focal points using several different methods. In one method, the mirror was placed on a plate of square or rectangular configuration. At three of the four corners of the plate, were placed screws which passed through threaded holes in the plate and which were coupled to rotatable bearings in the frame of the laser. When any one of these screws was turned, the entire plate would pivot on an axis defined by the center points at which the other two screws supported the plate. Because an edge of the plate rotated through an arc when any one of the screws was turned, the mirror would move closer to or further away from the dye jet. This would cause the focal point of the mirror to move closer to or further away from the dye jet also. Unfortunately, the fact that the mirror was pivoting also changed the angle between the incoming beam and the outgoing beam reflected from the mirror. A change in the angle, if not corrected, would move the focal point out of the dye jet or away from the position of maximum lasing efficiency. Therefore, to make such an arrangement work to translate the focal point, it was necessary to adjust all three screws so as to maintain the proper angle and move the focal point to the desired position. This was a painstaking and time consuming process, and was disadvantageous.
Another structure used in the prior art to translate the focal point of the mirror achieved true independent focal point translation without changing the angle between the incoming beam and the outgoing beam. This structure is referred to as the "translation stage". In such a device, the mirror was moved linearly by movement of a plate to which the mirror was attached along another plate to which the first plate was coupled by bearings. The mirror was also coupled to another support structure by several screws which could be turned to change the angle of the mirror if the angle between the incoming beam and the outgoing beam was to be changed. This support structure was then attached to the translation stage. This allowed the composite structure to be rolled linearly back and forth along a straight line relative to the laser base. The translation stage was coupled to the laser base such that the straight line of movement brought the mirror closer to or further away from the dye jet in a direct line between the mirror and the dye jet. This allowed the focal point of each mirror so mounted to be moved closer to or further away from the dye jet. Thus, when the focal point was translated relative to the dye jet, the angle between the input beam and the output beam was not changed since movement of the translation stage did not change the angle of the mirror.
The disadvantage with this approach is the expense of the translation stage. Since several mirrors have to have their focal points adjusted in a dye laser, the expense of the translation stage for each such mirror adds significantly to the cost of the product.
Thus, there has arisen a need for a simple, inexpensive apparatus for adjusting the position of the focal point of a curved mirror in a dye laser which allows the position of the focal point to be changed by a single movement which allows the position of the focal point to be moved along what is essentially a straight line.